Have you brought the whip antenna out of the case and extended it in a straight line? If it’s coiled inside, that is the first thing to do.
The next easiest way to improve signal strength is to provide the antenna you have with a ground plane.
There was a big discussion a little while ago - Improving RF signal strength emonTH emonTX
You can have the ground plane at either or both ends of the link.
There is provision for a socket (SMA - as the emonTx) on the emonTH pcb, but the case will need serious modification. With a socket, an external Yagi (directional) antenna would be possible.